1. Field of the Invention
The present invention relates to a liquid crystal composition useful for e.g. a display device, and a liquid crystal display device employing such a composition.
2. Discussion of Background
A liquid crystal device employing a guest host mode liquid crystal composition having a dichroic dye mixed to a host liquid crystal composition, is excellent in brightness or in the viewing angle, and thus is expected to be useful as a liquid crystal device for reflection type display to be used for portable data equipments.
The display quality such as the contrast of such a guest host mode liquid crystal device interrelates directly with the dichroism or the order parameter of the guest host mode liquid crystal composition to be used. Accordingly, a guest host mode liquid crystal composition which exhibits a high order parameter, has heretofore been desired. Especially, recently, a guest host mode liquid crystal composition which employs a new host liquid crystal composition of e.g. fluorine-type and which exhibits a high order parameter, has been desired, which is useful for active matrix addressing LCD.
In an active matrix addressing liquid display device employing e.g. a thin film transistor, the liquid crystal material is required to maintain stored charges for frame time. Accordingly, as such a liquid crystal material, it is essential to employ a material which has a high ability to maintain stored charges for frame time, i.e. a high holding-voltage ratio.
Most of such liquid crystal display devices are transmission type devices employing backlight. However, Personal Digital Assistant (PDA) which has been actively developed in recent years, is required to be driven for a long period of time with a limited battery, and its parts are required to have low power consumption. As a display device, preferred is a reflection type device of low power consumption type requiring no back light. Particularly effective is a guest host liquid crystal composition employing a dichroic dye, since its light utilization efficiency is high.
As such a liquid crystal material, instead of a conventional liquid crystal material containing cyano groups, it has been common to employ a fluorine-type liquid crystal material containing fluorine atoms or substituents containing fluorine atoms, such as fluoromethyl groups such as trifluoromethyl groups, or fluoromethoxy groups such as trifluoromethoxy groups, which usually scarcely contains ionic impurities. However, when a dichroic dye is added to such a liquid crystal material, it often happens that the holding-voltage ratio of the host liquid crystal is substantially impaired. A display intended to be used outdoors, such as Personal Digital Assistant, is required to have high electrical performance such as holding-voltage ratio, particularly with respect to durability against light such as direct sunlight. However, such a liquid crystal material having a dichroic dye incorporated, has a problem that there is a substantial decrease in the holding-voltage ratio after an irradiation test employing a xenon lamp or the like as the power source. For these reasons, it has been difficult to practically develop an active matrix addressing liquid crystal display device employing such as guest host liquid crystal composition.
Specifically, common azo dyes generally exhibit high order parameters, but they are inferior in stability against light as compared with anthraquinone dichroic dyes and thus have had a problem such as a decrease in the voltage-maintaining performance by exposure to light. Anthraquinone dyes are superior to azo dyes in their electrical characteristics to light, but the order parameters of the individual dyes tend to be inferior to azo dyes. Further, in a case where a composition having a black, brown or dark blue color is prepared by mixing a plurality of dyes of such azo and anthraquinone types, the order parameters of the individual dyes decrease in light absorption wavelength regions departing from the respective maximum absorption wavelength (.lambda.max). Accordingly, with the above composition having a color of black or the like, prepared by mixing dichroic dyes having various structures, the order parameter decreases in a wavelength region where light absorption spectra of the respective dichroic dyes overlap, whereby no adequate contrast of the entire guest host mode liquid crystal composition has been obtained.
For the above-mentioned reasons, it has been desired to practically realize a guest host mode liquid crystal composition and a liquid crystal display device showing a high order parameter and high light stability.